Cross-selling is the practice of selling additional products or services to existing customers. In a single transaction, this might involve upselling a higher-priced or more advanced version of a product the customer is already purchasing. For businesses that maintain long-term relationships with customers, however, cross-selling is a sustained effort to gain more sales from each customer over time. By identifying the needs and interests of their existing customer base, businesses can offer relevant and valuable products or services that enhance the customer’s experience and drive additional revenue.
The following are common types of cross-selling.

Complimentary Items
Products and services that compliment each other like coffee and donuts or software and training.

Seasons
Seasonal themes. For example, a back to school campaign might suggest shoes to a customer buying pens.

Data Driven
A platform might cross-sell items based on historical purchase data for similar patterns of shopping cart or page visits. For example, a site might suggest bicycle locks to someone who puts a bicycle in their shopping cart.

Promotions
Pitching things that are on sale. If a customer booked a trip to France last month, let them know when tickets to France go on sale.

Campaigns
A sales campaign to sell a particular product or service. For example, a telecom company might pitch colocation services to network customers.

Popular Items
Cross-selling popular items such as a bookstore that reminds customers that a best seller just went to paperback.

Experiments
Experimenting with cross-selling different items to see what works. For example, the food service on an intercity train might experiment with different food items such as sushi or pizza to see what sells.

Impulse
Things that people tend to buy on impulse such as candy bars.

Releases
Letting customers know when new products and updates are released. For example, a fashion brand that connects with customers to generate excitement for their Spring line.

Risk
Risk products such as an extended warranty or insurance.

Services
Wrapping products in services that deepen the relationship with the customer. For example, pitching an unlimited ebook service to people who buy an ebook reader. This results in monthly recurring revenue as opposed to a one time sale.